This invention relates to communication systems in which the contents of the message needs to be protected from undesirable interception by other than the party it is intended for, in which case sender and receiver commonly desire to practice some form of message signal manipulation to protect the confidential nature of the message.
Although not necessarily restricted to any one application, this invention will be found usefu in the practice of cellular telephone systems. Such systems, using public airwaves at specific known radio frequencies are particularly prone to unauthorized interception. Futhermore, as sender or receiver may operate from a moving vehicle, the possibility of signal fading is ever present, and one cannot count on being able to receive a continuous, uninterrupted signal. Possible signal discontinuity makes it impossible to employ some of the most powerful signal manipulation or encryption means*. These means operate on message blocks of specific length, and if during transmission part of a block is lost, block framing is lost and decoding by the receiver is impossible. Therefore, current state-of-the-art in cellular telephone encryption systems resorts to such well known schemes as audio frequency transposition, inversion, or partition and rearrangement by bands of frequencies. More secure systems, which rely on block recognition or synchronization are not available. FNT *U.S. Department of Commerce. FIPS PUB 46-1: Data Encryption Standard. FIPS PUB 81; DES Modes of Operation.